1. Field of the Invention
The present invention relates to a microcapsule magnetic migration display.
2. Description of the Related Art
A microcapsule magnetic migration display is developed and commercialized based on Japanese Patent Application Laid-open Nos. Hei 02-146082 and Hei 04-233581, which are filed by the applicant of the present application.
Japanese Patent Application Laid-open No. Hei 10-197908 discloses a microcapsule for a magnetic display and a magnetic display plate using the microcapsule, in which a low boiling point solvent and a high boiling point solvent are combined as a dispersion medium in the microcapsule, and the respective boiling point ranges and viscosities are defined, thereby attaining higher image forming and erasing speed, providing bright character and image having stability, and being excellent in durability.
Japanese Patent Application Laid-open No. 2001-83922 describes that, in addition to a mixed use of the low boiling point solvent and the high boiling point solvent as the dispersion medium in the microcapsule, particle diameters of white pigments and magnetic particles are substantially the same, and describes about a range of addition ratio of the magnetic particles and the white pigments with respect to the dispersion medium and a range of ratio of the white pigments with respect to the magnetic particles. Japanese Patent Application Laid-open No. 2001-83911 also discloses a thin display sheet in which an average particle size of the microcapsules is made small, and an improvement thereof is carried out so that, although the conventional image forming is performed from a display surface side, and the erasing is performed from a back surface side, in the thin display sheet of this invention, both the image forming and the erasing may be performed from the display front surface side.
Japanese Patent Application Laid-open No. 2003-195365 describes that, in addition to the mixed use of the low boiling point solvent and the high boiling point solvent as the dispersion medium in the microcapsule, the diameter range of the magnetic particles and a mixing ratio range are defined, and further describes that the image formed can be erased from the display front surface side.
Japanese Patent Application Laid-open No. 2006-227521 discloses a magnetic inversion display medium containing micro magnetic substances classified by colors through coloring of magnetic poles with different colors. This is a similar technology in that the microcapsule contains the magnetic particles, and particle size distribution of the contained micro magnetic substances, a ratio of an inner size of the microcapsule, and an outer size diameter range and the particle size distribution of the microcapsule are disclosed.
Conventionally, as a dispersion medium of a microcapsule-encapsulated dispersion liquid of the microcapsule magnetic migration display, it is thought that a dispersion medium which is a high boiling point solvent, and has a volume density at 15° C. of 0.91 to 1.10 g/cm3 is preferred. In order to enhance moving speeds of both particles of the magnetic substance and the non-magnetic substance in the microcapsule by adjusting the viscosity of the dispersion medium to be lower, the solvent having a lower volume density than the above-mentioned solvent may be mixed therein. However, the optimal low volume density of the microcapsule-encapsulated dispersion liquid is considered to be 1.0 g/cm3.
Even if the volume density of the microcapsule-encapsulated dispersion liquid is 1.0 g/cm3 or less, formation of the microcapsules is still possible. However, because the volume density is low, oil droplets dispersed during the formation of the microcapsules under the forming process are liable to float. Then, if the oil droplets or the microcapsules being dispersed in a process tank tend to move upward to a gas-liquid interface region in the tank during the forming process, resulting in causing of yields at the time of mass-production.
In any of Japanese Patent Application Laid-open Nos. Hei 10-197908, 2001-83922, and 2003-195365, as the dispersion medium of the microcapsule-encapsulated dispersion liquid, the low boiling point solvent and the high boiling point solvent are used as a mixture. However, the low boiling point solvent has an inflammatory risk, and a toxic risk due to inhalation of vapor of an organic solvent. Besides, in a production facility, an explosion-proof facility and an organic solvent recovery/treatment facility are required to install. In addition, the low boiling point solvent requires not only measure for safety, but also has a risk of causing an injury to an end user owing to such a hazardous substance contained in a final product, and thus being conceived as problems.
With regard to the high boiling point solvent, there are many concerns as follows. Phthalic ester, adipic acid bis(2-ethylhexyl), and the like are specified as class I designated chemical substances of Pollutant Release and Transfer Register (PRTR) system due to suspicion on environmental hormone. Besides, the component of Nisseki Hizol SAS-296 (manufactured by Nippon Oil Corporation), which is frequently used as a solvent for dispersing an oil droplet to be encapsulated, is specified as class II and class III monitoring chemical substances according to the Law Concerning the Examination and Regulation of Manufacture etc. of Chemical Substances (Japanese Chemical Substances Control Act), and is hardly decomposable, thereby being anxious about influence to environment.
Under the above-mentioned circumstances, with respect to the dispersion medium used for the microcapsule magnetic migration display, the supply of a dispersion medium, which is safe and harmless, has low toxicity, and achieves high yield mass production is required. Also required is a research to obtain an appropriate material by pursuing a technology for adjusting physical properties of available disperse medium.
In Japanese Patent Application Laid-open No. Hei 04-233581, a minimum particle size of the microcapsule is defined as 100 μm. However, as the particle size becomes smaller, distances between top portions and bottom portions of the respective microcapsules are made shorten, and hence degree of opacity of a layer consisting by the magnetic substances and the non-magnetic substances inside of the microcapsules become insufficient. As a result, a color contrast between the image formed on the display surface and a background portion becomes insufficient, and hence, conventionally, the lowest limitation of the particle size of the microcapsule must be set to 100 μm.
In Japanese Patent Application Laid-open No. 2001-83922, the particle size of the microcapsule is defined as smaller as 30 to 170 μm. In this particle size, the distance between the top portion and the bottom portion of the microcapsule becomes narrower, and hence it becomes difficult to hide a color of the magnetic substance layer or the non-magnetic substance layer existing at the bottom portion with the non-magnetic substance layer or the magnetic substance layer existing at the upper portion, resulting in a gradation of a gray color at the surface of the display. However, in this laid-open patent application, there is no disclosure about means for supplementing the critical insufficiency of the degree of the opacity.
In Japanese Patent Application Laid-open No. 2001-83922, too, there is described that both the image forming and erasing can be carried out by a magnetic field applied thereto from a front surface side. However, according to this invention, if the magnetic field for erasing the character or the image formed on the front surface side of the display is applied from the same front surface side, cohesion of light-absorptive magnetic particles, which are cohered for the formation of the image, are released to be diffused. As a result, the light-absorptive magnetic particles are mixed into one with the light-reflective non-magnetic particle layer existing as a base layer, and hence the display becomes a gray color when viewed from the display front surface side, resulting in being not a state called an erased surface. Specifically, in this invention, there is not described means for solving such a problem that the magnetic substance particles are spreadly diffused due to the magnetic field applied at the time of erasing the image formed from the front surface side, and hence a white color and a black color are mixed to cause the display surface to be gray.
In the invention disclosed in Japanese Patent Application Laid-open No. 2001-83922, there is described a conventional phrase, “the present invention is not limited to Examples,” to expand the technical scope of the invention. Further, components and their compounding ratio of the microcapsule-encapsulated dispersion liquid disclosed in example of this invention are completely the same with that of Japanese Patent Application Laid-open No. 10-197908. However, in Japanese Patent Application Laid-open No. 2001-83922, there is described as if the object to make thinner the microcapsule magnetic migration display sheet after its completion may be achieved by merely reducing the particle size of the microcapsule. As described above, however, unless otherwise any technical alternation is added to the structure of the microcapsule-encapsulated dispersion liquid, it is obvious that, compared with the display obtained in Japanese Patent Application Laid-open No. Hei 10-197908, quality of the microcapsule magnetic migration display sheet as disclosed in Japanese Patent Application Laid-open No. 2001-83922, namely, a performance to achieve further clarity of a white color degree and a black color degree by opacifying the white color and the black color with each other is further degraded as the particle size of the microcapsule is further reduced correspondingly, thereby being not yet achieved the effect which this invention seeks.
As described above, although Japanese Patent Application Laid-open No. 2001-83922 mentions that the object to make a thickness of the display sheet thinner may be achieved by merely reducing the particle size of the microcapsule, but in reality, the image forming quality of the display (for clarifying the image formed through the opacifying power between the light-absorptive magnetic particles and the light-reflective non-magnetic particles) is forced to be sacrificed by merely reducing the particle size of the microcapsule, thereby being far from a solution of the object to make the display sheet thinner.